In recent years auto focusing cameras have become more and more popular. One of the important performance characteristics for judging imaging quality of an auto focusing cameras is distinctness and even. FIG. 5 shows an aperture apparatus 100 disclosed in U.S. Pat. No. 4,425,501 for use in an auto focusing cameras. The aperture apparatus 100 comprises an integrated circuit chip 110 which has a plurality of detector pairs 111 positioned thereon, a transparent member 120, an aperture mask 130, a corrector lens 140, and a filter 150. The aperture mask 130 is constructed of an opaque material and is formed with a plurality of small apertures 131 defined therein. The transparent member 120 has a plurality of lenslets 121 formed thereon. When imaging, light from a remote object is incident upon the filter 150 and is first filtered, and then is transmitted through the corrector lens 140 so that it is substantially collimated. The light is then incident upon the aperture mask 130, but only the light incident on the small apertures 131 of the aperture mask 130 can pass through the aperture mask 130. Light passing through the apertures 131 is passed through lenslets 121 of the transparent member 120. This light is focused onto detector areas surrounding each detector pair 111 on the integrated circuit chip 110. The prior art apparatus 100 used the focusing of the lenslets to strengthen the light hitting the intensity of the detector areas and used the mask to prevent light from hitting the non-detector areas, thereby avoiding production of the spurious signal which can produce error in the circuit operation.
However, when the auto focusing camera works, a luminance from image is not always even. The luminance at the center of the image is generally strong and the luminance at the periphery of the image is so weak. This phenomenon of the decease in peripheral illumination has two cause vignetting and the Cos4law. In this prior art apparatus, a diameter of each aperture 131 is equal, whereas the illuminance of an image produced by the light passing through the corrector lens 140 gradually varies from strong at the center to weak at the periphery of the corrector lens 140. Since each of the apertures 131 of the mask is equal in diameter, light passing through the mask and incident on the lenslets 121 is strongest at the center of the circuit chip 110. The illuminance of an image at a center of the circuit chip 110 is brighter and the illuminance of an image at the periphery is dingier. This makes the whole image seem to be asymmetric and faint.
Therefore, an improved aperture apparatus for an auto imaging machine is desired which overcomes the disadvantages of the prior art.